Ethylene-vinyl acetate copolymers are well known commercial resins which have excellent properties e.g. clarity and heat sealability, which render them particularly useful in applications such as extrusion coating and packaging. Usually, the polymers are produced by copolymerizing ethylene and vinyl acetate in the presence of a free radical catalyst at elevated temperatures, e.g. from about 250.degree. to about 600.degree. F, and at very high pressures, e.g. from about 20,000 psi to about 50,000 psi. The reaction mixture withdrawn from the polymerization reactor is then reduced in pressure and separated in a high pressure separation zone into polymer product and unreacted monomer, the latter being recycled to the polymerization zone. The reduction in pressure of the total effluent from the reactor is accompanied by a sizable increase of the temperature of the effluent due to the inverse Joule-Thomson effect. It has been found that the severe conditions present in the high pressure separation zone are conducive to decomposition of the polymerized and/or unreacted vinyl acetate causing the formation of degradation products, e.g. acetic acid, which imparts an objectionable odor to the polymer product. Various methods have been disclosed in the prior art either for the removal of the malodorous contaminants from the polymer or the prevention of the formation of such contaminants from the polymer or for the prevention of the formation of such contaminants. For instance, in the latter category a technique has been disclosed in U.S. Pat. No. 3,509,115 involving reducing the temperature of the reactor effluent in the separation zone by introduction of at least part of the ethylene monomer feed into said separation zone. Although this method appears to function satisfactorily for its intended purpose, it has some economical disadvantages. Specifically, it is necessary to treat the unreacted monomer separated in the separation zone for removal of low molecular weight polymer by-product and also of polymer fines prior to introduction of the recycle monomer stream into the reactor. By the introduction of additional quantities of monomer into the separation zone, the size and therefore both capital and operating costs of equipment associated with the aforementioned necessary treatments are considerably increased.
It was found during the course of the experimentation leading up to the present invention that if no precaution is taken to prevent the formation of degraduation products in the process, these contaminants build up in the reaction loop as they are being returned to the polymerization zone with the unreacted monomer. After a relatively short time, e.g. 5 hours or less, the concentration of the contaminants will reach a critical level, causing an explosive decomposition of the copolymer formed in the reactor.
Explosive decomposition is known to occur in continuous free radical induced high pressure processes for the production of copolymers of ethylene and esters of fumaric acid or maleic acid. U.S. Pat. No. 3,466,265 discloses that this phenomenon is caused by the formation in the polymerization zone of so-called super molecules due to an intramolecular transfer mechanism. In the process of the patent, this formation is prevented by introducing into the reaction zone from 0.00005 to 0.00018 moles of a hindered phenolic compound per mole of total monomer feed. It is essential that the phenolic additive be present in the reaction zone during the formation of the polymer product. A serious drawback of introducing phenolic compounds into the reaction zone is that the compounds are free radical scavengers and therefore greatly inhibit the desired generation of free radicals from the initiators i.e. the peroxide catalysts. Consequently, a several-fold increase in the catalyst rate is needed to offset the inhibiting effect of the phenolic compound. The combined cost of the additive and the incremental catalyst per unit weight of polymer is quite substantial and could be prohibitive, if the process of the patent was to be modified to the production of ethylene-vinyl acetate copolymers, which are relatively low priced resins.
It is therefore an object of the present invention to provide an improved process for the production of ethylene-vinyl acetate copolymers, wherein explosive decomposition of the polymer is prevented.